Energetics

Question 1

What energy change is breaking bonds associated with?

Answer 1

Energy is taken in to break bonds ==> endothermic reaction

Question 2

What energy change is making bonds associated with?

Answer 2

Energy is released to make bonds==> exothermic reaction

Question 3

What are some uses of thermochemistry?

Answer 3

Measuring and comparing the energy values of fuels
Calculating the energy requirements for industrial processes
Working out the theoretical amount of energy released/taken in a reaction
Predicting if a reaction will take place or not

Question 4

What is an endothermic reaction?

Answer 4

One with an overall positive enthalpy change => energy in breaking bonds> energy out making bonds

Question 5

What is an exothermic reaction?

Answer 5

One with an overall negative enthalpy change => energy in breaking bonds< energy out making bonds

Question 6

If a reversible reaction is endothermic one way, what type of reaction is the other way?

Answer 6

exothermic

Question 7

Give two examples of exothermic reactions

Answer 7

Combustion of fuels
Neutralization

Question 8

Give an example of an endothermic reaction.

Answer 8

Thermal decomposition

Question 9

Define enthalpy change; what symbol is used to represent it?

Answer 9

Energy change of a system at a constant pressure presented by delta H

Question 10

What are the standard conditions?

Answer 10

100kPa/ 1atm pressure
298 K/ 25 degrees Celsius temperature

Question 11

What does “ in standard state” mean?

Answer 11

The state an element/ compound exists at standard conditions (100kPa, 298 K)

Question 12

Draw an enthalpy change diagram for an endothermic reaction, and one for an exothermic reaction.

Answer 12

Look for these on PMT

Question 13

Define standard enthalpy of formation

Answer 13

The enthalpy change when one mole of a compound is formed from its constituent elements in standard conditions, with reactants and products in their standard states

Question 14

Give an examples of an equation which represents standard enthalpy of formation

Answer 14

H2 (g) + 1/2 O2 (g) => H2O (Il)

Question 15

Define standard enthalpy of combustion

Answer 15

The enthalpy change when one mole of a substance is burnt completely in oxygen in standard conditions, with reactants and products in their standard states.

Question 16

Give an example of an equation which represents standard enthalpy of combustion.

Answer 16

C (s) + O2 (g) => CO2 (g)

Question 17

What is the difference between heat and temperature?

Answer 17

Heat is the sum of all particles’ energy, therefore it is affected by the amount of substance; temperature is related to the mean kinetic energy of the particles in a system, so is independent of the number of particles present.

Question 18

How can you calculate enthalpy change from experimental data?

Answer 18

Use the equation Q =mcT, where m is the mass of the substance being heated (usually water), c is the specific heat capacity of that substance (water’s SHC = 4.18 g J^{-1} K^{-1} and T is the change in temperature

Question 19

Draw a simple calorimeter

Answer 19

Look for this on PMT

Question 20

How can a calorimeter be made more accurate?

Answer 20

1. Try reducing heat loss by adding mineral wool around the beaker
2. Add draught screens at the sides
3. Or add a lid on top of the beaker if not present

Question 21

What is a flame calorimeter; how does it differ to a simple calorimeter?

Answer 21

Reduces heat lost to the surrounding to give more accurate results: has a spiral chimney made of copper, an enclosed flame and the fuel is burnt in pure oxygen, not in air.

Question 22

How would one measure the enthalpy change for a reaction occurring in (aq)?

Answer 22

1. Use an expanded polystyrene cup as a calorimeter (good insulator- which reduces heat loss)
2. Heat is generated in the solution; measure this temperature change.
3. Take heat capacity of solution to be 4.18 and density of solution = 1 gcm^{-3`}

Question 23

What can one use to make experimental determination of enthalpy change of reaction more accurate?

Answer 23

Cooling curves

Question 24

What is Hess’s Law?

Answer 24

states that the enthalpy change for a reaction is the same regardless of the route taken

Question 25

What is the enthalpy of an element?

Answer 25

The enthalpy of all elements in their standard states (the states in which they exist at 100 kPa and 298 K) is defined as 0

Question 26

Define bond dissociation enthalpy.

Answer 26

The enthalpy change required to break a covalent bond, with all species in the gaseous state; differs for the same bond type in different molecules

Question 27

Define mean bond enthalpy.

Answer 27

Average value(across different chemical environments) for the bond dissociation enthalpy of a given bond.

Question 28

Why may experimental methods for enthalpy determination not be very accurate?

Answer 28

1. Heat is lost to the surroundings
2. Not in standard conditions
3. Reaction may not go to completion

Question 28

Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?

Answer 28

Bond enthalpies are a mean for the same bond across different molecules; standard enthalpy of combustion and formation apply just to that molecule, therefore they are more accurate